The present invention relates to a semiconductor device, particularly to the structure of the capacitor portion of a DRAM (Dynamic Random Access Memory) and the, method of manufacturing the same.
Recently the cells of DRAMs have become smaller. Particularly the dielectric film of the capacitor portion thereof is required to be thinner. For example, a technical article published in the Spring 1991 Applied Physics Society Journal 28p-V-5, "Electrical Conduction in a Very Thin Oxide-Nitride-Oxide Layered Film," authored by Kobayashi and others, discloses a technical problem in thinning the ONO (Oxide-Nitride-Oxide) film and the method of solving the same.
The technical problem disclosed in the literature is that when the nitride, i.e., the silicon nitride film is thinned, the breakdown voltage thereof is abruptly lowered. The literature teaches to thin the oxide under the nitride film, i.e., a naturally formed silicon oxide film, as a method of solving the problem. In order to apply the method disclosed in the above literature, the natural oxide (native oxide) under the silicon nitride film should be removed and the substrate should be protected from another natural oxidation. That is, it is preferable to adopt a system in which the removal of the native oxide film and the formation of the silicon nitride film can be successively performed while as the ONO film is shut off from the outside air. The system is, for example, a Load Lock Low-Pressure Chemical Vapor Deposition (referred to as an LPCVD hereinafter) device equipped with an in situ HF vapor cleaning chamber.
The inventors brought the thickness of the native oxide film near to zero as close as possible and thereafter formed the silicon nitride film using the above mentioned system. The inventors learned that when the native oxide film is thinned to an extreme degree employing this system, a silicon nitride film selectively grows on the clean surface of the silicon while it hardly grows on the silicon oxide film later. Accordingly, a polycrystalline lower electrode is oxidized at the tip end portion thereof when an oxide film is formed on the silicon nitride film, which results in a problem of lowered capacitance of capacitors. There occurred also a problem of deteriorated electric breakdown characteristic.
It is an object of the present invention to provide a method of manufacturing a semiconductor device including a capacitor which has a high capacitance.
The above object is generally achieved according to the present invention by a method of manufacturing a semiconductor device having a capacitor, which comprises the steps of forming a silicon oxide layer on a semiconductor substrate, forming a first silicon nitride layer on the silicon oxide layer, forming; a polycrystalline silicon layer as a lower electrode layer of the capacitor on the first silicon nitride layer, subjecting the polycrystalline silicon layer to the oxygen-containing outside atmosphere so as to form a native oxide layer on the polycrystalline silicon layer, removing the native oxide layer in a processing system shut off from an oxygen-containing atmosphere to expose the polycrystalline silicon layer to an inactive gas atmosphere, forming a second silicon nitride layer on the exposed polycrystalline silicon layer in the processing system, subjecting the second nitride layer to the oxygen-containing atmosphere so as to form a capacitor oxide layer on the second silicon nitride layer, the second silicon nitride layer and the capacitor oxide layer working as a dielectric layer of the capacitor, and forming an upper electrode layer of the capacitor on the capacitor oxide layer.